The present disclosure relates generally to locating and shimming one part relative to another part, such as parts constructed and assembled for aircraft or ships.
Determinate assembly (DA) is a method of aligning parts using mating physical features. Typically, coordinating holes which are placed on each part or structure are used to take advantage of the ability to install temporary fasteners to hold the parts together. Determinate assembly holes are typically installed on a part or structure during production or via a computer numerical control (CNC) mill onto a part that is properly indexed in a jig so hole installation is accurate. There are three major challenges to drilling and using DA holes, particularly in large, built-up structures such as wings. First, DA holes are accompanied by inherent reference or indexing tolerance. Tolerance build-up, especially in large parts or in assemblies, can cause DA to be ineffective, particularly when used in large assembly indexing. Second, DA hole placement on parts may not be feasible for built-up structures or for parts that cannot be indexed properly or easily in a CNC mill (i.e., a wing, wing panel, or fuselage section). For large assemblies where parts are placed relative to one another, DA holes cannot be placed ahead of time because the correct location is not yet known. Lastly, large surfaces (such as a fuselage, wing panel, etc.) can flex during installation or the surface definition may vary considerably and deviate from engineering nominal. Any drill jig applied to the surface without consideration of the as-built surface variation, results in holes with the additional error from jig placement on the as-built surface.
Currently, assembly of parts on large structures is achieved via monument locating jigs that hold parts in place while they are attached. These jigs are expensive to build and must be maintained at considerable cost. Typically, they are the largest non-recurring costs in new airplane development. Large, monument jigs also present a financial bottleneck for future rate increases or incorporation of new airplane derivatives. For the placement of strut fittings, additional errors in placement occur when the jig is adjusted to try to minimize shim thickness over three fittings installed in a monument drill jig. Existing, manually adjustable drill jigs have been tried before. Their primary drawback is that they require a metrology system that provides real-time positioning feedback, limiting the types of metrology systems that can be used. A second drawback is that these solutions do not take into account the effect of surface variation on hole location, thus inducing error in their hole positioning. Also, they are inefficient in positioning for many hole locations since each must be adjusted by hand. Finally, errors associated with the locate/drill jig are repeatedly transferred to every airplane.
In addition, the parts of an assembly are sometimes required to be joined together with an accuracy that is within a specified tolerance. For example, in the aerospace industry, some parts may be required to be assembled together with less than a 0.005 inch gap between them. When the gap exceeds the specified tolerance, a shim or similar filler may be inserted into the gap in order to assure a within-tolerance fit between the parts. Shims can be used to fill voids discovered during an assembly process. Voids are typically formed by the misalignment of parts during assembly or by the incorrect manufacture of the parts being assembled. Although mostly used on an informal basis during manufacturing, some shims are called out on drawings as part of the manufacturing process. Filling voids between mating surfaces on assembled parts results in a more structurally sound assembly. Shims are used throughout the aerospace industry to compensate for part variation due to the complex aerodynamic shapes of various assembled parts.
One method of filling the gaps between mating parts, sometimes referred to as predictive shimming, involves scanning the interfacing part surfaces in an attempt to predict the exact shape of the gap or void between these surfaces. The parts of the assembly are virtually fitted together and a shim is fabricated based on the virtually predicted relationship between the parts. In accordance with one method, digitally defined shims are created for joining interfaces at each part surface to join parts of an arbitrarily configured assembly. The method takes two or more digital surfaces that define independent joining surfaces and after establishing a relationship between them, a resultant virtual solid mass is created that exactly fills the void between the mating surfaces. One advantage of the disclosed method is the ability to determine part-to-part relationships of an arbitrary “as built” part configuration at the time of assembly, rather than predict relationships in advance.
There is room for improvements in methods and apparatus for locating and shimming one part relative to another part when the parts are to be aligned using mating physical features such as drilled coordination holes.